The paper presents the thermal performance comparison between two types of solar air collectors, with and without porous absorber material. In order to simulate the transient processes which occur in the collectors two mathematical models were developed. Each model consists of a set of time dependent equations which describe the energy balance for the main components of the collector considering time-dependent thermo-physical properties and heat transfer coefficients. The theoretical models were validated by experimental data obtained from measurements performed in different meteorological conditions characteristic to the city of Bucharest – Romania (latitude 44°26′ N, longitude 26°6′ E). Good agreement between the simulation results and experimental data was achieved.